A large proportion of individuals with alcohol and illicit drug abuse and dependence in the community also have mental disorders and, vice versa, a large proportion of individuals with various mental disorders also have substance disorders. Nevertheless, the majority of these individuals do not seek treatment. Even among those who receive treatment the outcomes of treatment for comorbid disorders are often worse than for non-comorbid mental or substance disorders. However, we know very little about the types of services that these individuals use or their unmet need for treatment. We also do not know whether they face the same barriers to accessing services as individuals with non-comorbid mental or substance disorders. If we are to improve these individuals' access to appropriate mental health and substance disorder services we have to learn more about their service use patterns, their unmet needs and the barriers they face in accessing services. In the proposed project we plan to use two sources of data from large and recent nationally representative surveys of the US general population to obtain this much needed information. We will use data from the longitudinal National Epidemiological Survey of Alcohol and Related Conditions (NESARC) waves 1 and 2, and the annual cross-sectional National Survey of Drug Use and Health (NSDUH) for years 2004-2008. These data provide an unprecedented opportunity to examine service use and barriers to care in population subgroups (e.g., racial minorities) that could not be adequately examined in previous studies due to small sample sizes. In Aim 1 of the study we examine patterns of service use and compare participants in these surveys with comorbid and non-comorbid disorders to determine whether they use the same types, mix or amount of mental health or substance disorder services. We also compare these groups regarding perceived unmet need for care. In Aim 2 we compare the comorbid and non- comorbid groups regarding the types and prevalence of specific reasons given for not using needed services and compare the reasons for not using mental health vs. substance disorder services. We capitalize on the longitudinal design of the NESARC data to assess whether some reasons are more persistent than others over time and whether some reasons are more strongly associated with not using services at a future time than other reasons. In Aim 3 also we use the longitudinal NESARC data to assess whether use of certain services or a certain mix of services at baseline is associated with a greater reduction in use of substances, abstinence or better social functioning at follow-up. We will also examine the effects of race-ethnicity and gender in these analyses. Beyond their scientific merits, the results from this study will be helpful in designing services that are more responsive to the needs of patients with comorbid mental and substance disorders. The data will also help in designing public campaigns that aim to improve access to services by reducing the stigma and other attitudinal and structural barriers to care. PUBLIC HEALTH RELEVANCE: The majority of individuals with comorbid mental and substance disorders do not seek treatment and among those who do seek treatment, the outcomes of treatment are often worse than for non-comorbid disorders. To help improve access to services and outcome of care for this group of individuals this study will use data from two large US epidemiological surveys to answer three questions: (1) whether individuals with comorbid disorders use the same types, amount and mix of services as those with non- comorbid disorders and whether they experience the same degree of unmet need; (2) whether individuals with comorbid disorders face the same barriers to mental health and substance disorder services as those with non-comorbid disorders and which barriers are more persistent and cause greater hindrance; and (3) whether a certain type or mix of services is associated with better clinical and social outcomes in individuals with comorbid disorders.